In wireless communication networks, uplink and downlink traffic must be duplexed in such a way that they can be separated from each other. The most common methods of doing this are Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD), i.e. separating the uplink and downlink signals in frequency and time, respectively. Each of these methods has advantages and disadvantages over the other.
TDD has the advantage over FDD that required resources in the uplink and downlink can be assigned in a more flexible manner. Further filters, which are costly and potentially space-requiring, are not needed, and channel reciprocity may be exploited for channel and path gain estimations.
To exploit the resources of the wireless network as flexibly as possible, it should preferably be possible to vary the time used for uplink and downlink traffic, respectively, in order to accommodate varying traffic needs in up and downlink over time. This means that at any given time, it is possible that some cells will use uplink resources and some cells will use downlink resources. This leads to increased interference between cells, both between base stations and between mobile terminals.
In particular, the interference between base stations is a problem, since the path loss between base stations is often very low in comparison with the path loss between mobile terminals. Downlink traffic transmitted from one base station will interfere with uplink traffic received in another base station, since the former will typically have much higher power than the latter.
Traditionally, methods of avoiding such interference have involved resource allocation or scheduling.
One such method to avoid the interference between base stations is obviously to assign uplink and downlink resources in any two cells such that they never overlap. Such a solution is described in Lindström, M.: “Resource Allocation for Asymmetric Traffic in Time Division Duplexing Mode Cellular Networks” (2003, Licentiate thesis, 2003. In a system with many cells this may be difficult to achieve, since the resource assignments in the cells are interconnected. Also, the flexibility of assigning different data rates in adjacent cells depending on the needs is lost.
Another solution, disclosed in U.S. Pat. No. 6,334,057, that allows overlaps of uplink and downlink traffic in two adjacent cells, is to schedule mobile terminals that are close to a base station, instead of mobile terminals farther away, during any potential uplink-downlink overlap between any two cells.
Tardy, Grøndalen, Vezzani: “Interference in TDD based LMDS systems” IST Mobile and Wireless Communications Summit, Thessaloniki, 17-19 Jun. 2002, describes a different approach based on interference cancellation techniques. The base station that experiences interference can obtain a copy of the data transmitted by the interfering base station and use this copy to cancel interference. No concrete suggestions are given for how to achieve this in a transport efficient manner, or for a cellular system with at least partially distributed Radio Resource Management (RRM).